1. Field of the Invention
The present invention relates to injection molding machines which process plastic materials, and, more particularly, to the hydraulic controls for injection units of injection molding machines which serve to plastify plastic raw material and to inject it under pressure into an injection molding die.
3. Description of the Prior Art
For the production of high quality injection-molded parts with minimal variations in size and weight, it is important that the operation of the injection unit be controlled with precision and consistency over the entire production run. The injection unit, on the other hand, must be capable of accommodating a variety of operating conditions, in terms of plastic materials composition, injection rates, and injection volumes.
It is also important that the injection unit as a whole be of a compact and simple design which offers the neccessary operational reliability and adjustability in combination with a high longevity. Such an injection unit is known from my U.S. Pat. No. 3,833,204, particularly as shown in FIG. 2 thereof. This prior art injection unit features two parallel guide rods which support the injection unit and which, in turn, have one extremity attached to a stationary component part of the die closing unit and the other extremity either supported on the machine base or extending from the die closing unit in a cantilever fashion. The two guide rods carry two supporting bridges in a tandem arrangement, one behind the other. The front supporting bridge carries a plastification cylinder in a parallel central relationship to the guide rods, and the rear supporting bridge carries a cooperating plastification screw which is rotatable and axially movable inside the plastification cylinder. The rotation of the plastification screw is produced by a hydraulic rotary drive which is likewise carried by the rear supporting bridge.
An operating cylce of the injection unit consists essentially of a plastification stroke and a subsequent injection stroke. During the plastification stroke, the plastification screw rotates, as granular raw material is being fed into the rear of the plastification cylinder, in the area where the latter is seated in the front supporting bridge. The forcible advance of the raw material by the plastification screw pushes the latter rearwardly, until the required quantity of raw material has accumulated in front of the plastification screw. At that point, the unit is ready for the injection stroke which consists of a forcible forward movement of the plastification screw inside the plastification cylinder, thereby injecting the plastified raw material into the injection molding die, through an injection nozzle at the forward extremity of the plastification cylinder.
Both supporting bridges form hydraulic cylinder assemblies where they surround the guide rods, for the control of the axial movements of the supporting bridges on the guide rods. A movement of the front supporting bridge produces a corresponding axial movement of the entire injection unit, including the plastification cylinder, thereby giving access to the nozzle of the latter and the sprue channel of the injection molding die. The axial movements of the rear supporting bridge produce movements of the plastification screw relative to the plastification cylinder. The movements are controlled by the cylinder assemblies of the rear supporting bridge, the pistons of these cylinder assemblies being hollow sleeve-like extensions of the front supporting bridge.
The hydraulic controls for such an injection unit are automated to the extent that they utilize control inputs of predetermined values in the form of electronic signals which produce continuous adjustments of the pressure and flow rate of the hydraulic fluid which is delivered to the cylinder assemblies, especially the cylinder assemblies of the rear supporting bridge of the above-described injection unit.
In order to obtain the desired adjustments in fluid pressure and flow rate, the electronic input signals are fed to a suitable proportional-response valve which, depending on its connections in the hydraulic control circuit, serves as a throttle valve controlling the fluid flow rate or as a bypass valve controlling the fluid pressure. Both types of proportional-response valves are known from the prior art. A practical application of a proportional-response flow control valve and a proportional-response pressure control valve in connection with an injection molding machine is disclosed in my U.S. Pat. No. 4,020,633.
It is also known from the prior art to equip an injection unit with hydraulic controls which include as their main control component a servo-valve which features pressure transducers in the hydraulic supply lines as part of an electronic feedback circuit and which receives its input signals from an electronic computer, for example. Systems of this type are known as process control systems and they are normally more complex and more expensive than control systems which utilize proportional-response valves with presettable input values. The electronic servo-valve circuit, on the other hand, compensates automatically for any pressure losses or leakages in the hydraulic controls, thanks to the feedback connection between the supply lines of the hydraulic drive assembly and the input signal generator. Its preferred application is therefore found in connection with injection molding machines and injection units which have to meet very high product quality standards. Detailed performance data of a machine with a servo-valve circuit are reported in the periodical "Plastverarbeiter", vol. 9, (pp. 475-479.)